This invention relates to powdery compositions having a high rubber content which are useful as impact modifiers. This invention also relates to plastics additive polymer particle dispersions having a high rubber content which can be spray dried into a powder form. This invention further relates to a process for preparing powdery compositions having a high rubber content which are useful as impact modifiers. This invention even further relates to polymeric compositions that include a polymeric component and one or more high rubber impact modifier compositions.
Numerous molded articles and films are manufactured from one or more of a variety of polymeric resins. Often times, these resins by themselves are brittle and do not possess suitable impact strength required by the end use for which they are made. To overcome their shortcomings, these resins, especially poly(vinyl chloride), are generally blended with plastics additives which improve impact strength. Such plastics additives are typically known in the industry as impact modifiers and are typically supplied in a powder form.
Many impact modifiers which are useful in many resins are based on core-shell polymer particle compositions. These core-shell polymer particles typically contain a soft rubbery polymer phase (e.g., core) surrounded by a hard polymer phase (e.g., shell). The weight ratio of the rubbery phase to the hard phase is usually as high as possible to make the impact modifier as efficient as possible at improving the impact strength of the thermoplastic. However, when the weight percentage of the rubber phase exceeds 90 weight percent (“high rubber”) based on the weight of the total core-shell polymer particle, it has previously been unrealizable to isolate such high rubber content particles as a free-flowing powder at practical isolation rates.
Spray drying is an economical, safe and desirable means of isolating dispersions of core-shell polymer particles as free-flowing powders. During this process an aqueous dispersion of core-shell polymer particles is atomized in a chamber containing heated air, water is removed, and the core-shell polymer particles are aggregated into dry powder particles. Unfortunately, there are several problems associated with spray drying polymer particle dispersions containing a high rubbery phase content above 90 weight percent. These problems include: (1) sticking of the particles to the chamber walls of the spray dryer; (2) bridging of the particles over conveying lines entrances; and (3) unacceptable powder flow characterized by aggregation, clumping, and flow interruptions.
One of the methods of solving this problem as been suggested in U.S. Pat. No. 4,278,576. In this Patent, Goldman combines stearate coated calcium carbonate particles (“flow aid”) with high rubber impact modifier particles before or during the spray-drying step to provide free flowing impact modifier powders. While Goldman exemplifies an 88 weight percent rubber-content impact modifier powder can be provided by co-spray drying with 7 weight percent by weight of flow aid, there are many advantages associated with increasing the rubber content and reducing the flow aid content. For example, one advantage of having a powdery impact modifier which has a rubbery content greater than 90 weight percent is its improved effectiveness to increase the impact strength of matrix resins. As well, it is also advantageous to eliminate or reduce the required flow aid to less than 7 weight percent to further improve both the impact modifier's effectiveness as well as its economic efficiency.
The problem addressed by the invention is to provide high rubber impact modifier powder compositions in which the weight fraction of the rubbery phase is greater than 90 weight percent. We have now found that high rubber impact can be readily spray dried to a powder when the impact modifiers contain core-shell polymer particles which are provided as two or more populations of polymer particles wherein the mean particle diameters of the two populations vary by at least 50 percent. We have also suprisingly found that these new high rubber core-shell polymer impact modifiers do not necessarily require any flow aid to provide a spray-dried powder compared to similar core-shell polymers having a single population of particles. We have further suprisingly found that these high rubber core-shell polymer impact modifiers require less flow aid to provide compaction-free powders compared to similar core-shell polymers having a single population of particles.